Digital time stamps, like their paper counterparts, are used to certify that a specific document has not been modified since a specific date. A digital time stamp includes a hash value calculated from the document to be time stamped, the current time at the time of the timestamp, and a digital signature signing both the hash value and the current time.
The digital signature and hash value are what make a timestamp secure (i.e., they ensure the authenticity of the timestamp). Digital signatures generally rely on public key cryptosystems such as the Rivest-Shamir-Adleman (RSA) public-key cryptosystem. Hash values are calculated using hash functions such as SHA-1 (Secure Hashing Algorithm 1) and MD5 (Message Digest 5), which map entire documents into fixed-bitlength numbers. If the integrity of either the hash function used to produce the hash value or the cryptosystem used to produce the digital signature is compromised, the timestamp's integrity is ruined as well.
Many real-life applications of computer technology depend on the long-term storage of data. An example of this is the U.S. Internal Revenue Service's use of computers to store information regarding taxable gifts made over a person's lifetime. For most people living in the United States, gift taxes are not calculated or paid until death, so any information regarding taxable gifts must be maintained over a person's lifetime. In terms of probable advances in computer technology and cryptanalysis, a person's lifetime is like an eternity-it is impractical to assume that the cryptosystems available today will provide any measure of security in 50–70 years. Thus, a need exists for a timestamping mechanism that can adapt to changes in technology to provide a secure timestamp over a long duration of time.